The conversion system has been described herein with reference to its use in reproducing digitally coded musical information that has been generated in accordance with the format of the Musical Instrument Digital Interface (hereinafter referred to as "MIDI") standard for communication of serially encoded musical data, as standardized by the International MIDI Association and agreed upon by the manufacturers of keyboard synthesizers. MIDI is a standard for transmitting digital signals originated by keys pressed on a synthesizer keyboard and which may be carried over a pair of wires to one or more additional synthesizers which then may all play in concert. The MIDI signals are transmitted at a serial data rate of 31.25 kilobaud and may be processed by various algorithms in computers to allow for transposition, deletion of certain signals, repetition at a fixed time period (echo) or other processes. These signals are typically stored on various computer-controlled media such as hard discs, floppy discs or digital tape. The devices used to record and reproduce MIDI data are typically known as sequencers.
One of the principal functions of known sequencers is to associate with each discrete MIDI data group or "event", an additional code word giving the time of occurence of the event. This allows a large number of events to be stored in the limited extent of a computer memory or floppy disc. Upon reproduction, the sequencer evaluates the time codes and thus recreates the original rythmic structure of the music. In effect, each event is "time stamped" and the entire musical piece must be committed to memory as a "batch". Heretofore, systems have been devised wherein the batched, time-stamped event data groups have been recorded on inexpensive magnetic tape by tape recorders but this has involved batch dumping, rather than real time recording on the tape. It has also required the use of large random access memory, or RAM, units having capacities equal to or larger than the longest musical data to be recorded on the magnetic tape. Examples of time-stamped, batch type tape recording of musical signals may be seen in U.S. Pat. No. 4,614,983, granted to Ryuuzi Usami on Sep. 30, 1986 and U.S. Pat. No. 4,615,024, granted to Minoru Usui on Sep. 30, 1986.
Other systems have been devised for processing musical event data information so that it can be recorded and played back from magnetic tape. These systems, termed imaging systems or sampling systems, involve complex multiplexing arrangements wherein all keys of a keyboard are repetitively sampled at high rates to determine whether or not the key has been actuated. Such continuous sampling or imaging systems in addition to being complex and expensive to produce are susceptible to errors in reproduction due to the lack of coincidence between the actuation of various keyboard keys and the precise times that the keys are being sampled. Sampling, or imaging, can only effectively be done on one 88-key keyboard at a time, and the rate of sampling for each key in such a case is about 25 times per second. If more keys were to be sampled, the rate of sampling would have to be correspondingly slowed down, introducing greater timing errors. In addition, imaging systems are limited in the amount of information that can accompany the base signal that signifies the turning on or off of a given note. In effect it is a single dimensional system of limited applicability as compared to the multi-dimensional MIDI system. Examples of patents showing imaging or sampling systems are U.S. Pat. No. 3,604,299, granted to Edward J. Englund on Sep. 14, 1971 and U.S. Pat. No. 4,104,950, granted to William S. Finley on Aug. 8, 1978.
The upper end of the frequency response range of inexpensive cassette recorders, players and tapes is about 8 kiloherz, with 4-7 kiloherz being the usual upper range within which reliable recording occurs. As indicated earlier, the MIDI standard provides for a serial data rate of 31.25 kilobaud. Accordingly, direct recording of MIDI signals by such recorders on inexpensive cassette or open reel tape is impossible since the frequency responses of the recorder and tape are too low to properly record the serial data.
It is, therefore, a primary object of this invention to provide an inexpensive system for converting high transmission rate serial format digital data byte groups to lower transmission rate serial format digital data byte groups, and vice versa, to facilitate real time recording of such data on, and playback of such data from, an inexpensive recording medium.
Another object of this invention is to provide an inexpensive accessory to MIDI-equipped synthesizers, allowing the recording of MIDI event signals in their proper real time rythmic and melodic manifestation.
Further objects and advantages of this invention will become apparent as the following description proceeds.